Means exist for detecting the thermal level to which materials have been subjected. Detectors are known whose functioning is based for example on the variation in fluidity of a coloured matter, of which the progression by capillarity in a support gives a visual indication of exceeding a given temperature. Detectors using a different principle are also known, where an enzymatic reaction is translated by the variation in coloration of a surface of which the extent also indicates exceeding a thermal threshold. Such detectors are generally placed in the immediate proximity of the material to be monitored or thereon.
Such detectors present drawbacks: they are expensive, which does not allow use thereof for verifying the freshness of a food product, for example presented in the form of individual portions. Furthermore, these detectors, applied on a product, always indicate the exceeding of a superficial temperature or one in the proximity. In addition, they are of a certain fragility and cannot without drawbacks be immersed in a liquid or be subjected to compressions or inevitable shocks in the handling and transport operations. Furthermore, their structure does not allow them to be driven into the material or product to be monitored.
The evolution of eating habits of populations living in so-called developed countries is more and more oriented towards prepared dishes, cooked in vacuo for example. The life duration of such foodstuffs is a function of several parameters of which the most important appears to be the temperature which, when it lies below a determined maximum value, limits proliferation of bacteria. The same applies to the preservation, with maintenance of innocuosness, of certain drugs, including vaccines and sera. The examples of application set forth hereinabove are not limiting.